The roughness factor of barrier-type anodic oxide layer with low electrical conductivity was evaluated using electrochemical measurement in electrolyte with redox couple. To evaluate the surface roughness factor, the surface of barrier-type oxide layer was covered with thin layer of platinum by sputtering. From the limiting diffusion current method the surface roughness factor of barrier-type Al2O3 estimated to be 1.03. This factor is in good agreement with the results of the cross-sectional characteristics by using transmission electron microscope. (C) 2003 Elsevier Science B.V. All rights reserved.